Immune function is highly dependent upon costimulatory signals where their role has become apparent in the study of tumor infiltrating lymphocytes; expression of PD-L1 can engage PD-1 leading to T cell dysfunction with unchecked tumor growth. Similarly, there is a central role of T cell costimulatory signals in autoimmune disease particularly with activation of autoreactive T cells and induction of epitope spreading which has become evident with the emergence of autoimmune toxicities after blockade of coinhibitory signals in patients with cancer. Moreover, genetic variants identified in the costimulatory receptor CD226, and its ligand CD155 (shared with TIGIT) are strongly associated with risk of MS, indicating that this pathway maybe central in MS autoimmunity. These studies have led us to more deeply examine the TIGIT/CD226/CD155 and PD-1/PD-L1 pathways in autoimmunity versus their opposing roles in cancer followed with building regulatory networks in T cells by first analyzing interactions between these costimulatory pathways pairwise and then integrating co-stimulatory/co- inhibitory T cell signals. The uniqueness of this grant resides in 3 areas: 1) in vitro and in vivo model systems to evaluate costimulation in autoimmune disease; 2) availability of unique reagents to inhibit costimulation; and 3) PPG investigators to translate basic research to the clinic. In Project 1, we will examine the role of TIGIT/CD226 pathway in regulating T cell responses, investigating the hypothesis that TIGIT has dual roles in regulating T cell responses, cooperating with other co-inhibitory molecules to regulate effector T cell and Treg responses. We will dissect the roles of TIGIT in effector T cells and Treg in experimental inflammatory conditions. In Project 2, we hypothesize that loss of TIGIT expression or function in MS drives autoimmune responses, while in tumors, increased TIGIT expression or signaling allows tumors to escape immune surveillance. We will determine the role of TIGIT/CD226 in regulating CD4+ T cell activation in human inflammatory disorders. Secondly, we will investigate the role of TIGIT/CD226 on the function of Treg populations in MS, hypothesizing there are defects in the function of TIGIT+ Tregs in MS patients inducing dysfunctional Th1-like Tregs. In Project 3, we will use an integrative approach to understand mechanisms of costimulation. Our goal is to define genetic circuits underlying interactions between the costimulatory coinhibitory receptors PD-1 and TIGIT. We will determine how these costimulatory and coinhibitory receptors interact to regulate CD4+ T cell functions, and from that infer a transcriptional network mediating T cell function. Finally, we will test the roles of candidate regulators in the network underlying the activities of co-stimulators and co-inhibitors in CD4+ T cells. These studies will provide mechanistic insights into how TIGIT and PD1 exert their critical immunoregulatory functions in regulating T cell tolerance and autoimmunity. Understanding mechanisms inducing blockade of co-inhibitory signals that have had such dramatic effects in treating cancer may provide insight into therapeutic approaches for blocking co-stimulatory signaling in autoimmune diseases.